Electrolytic cladding



ELECTROLYTIC CLADDING Bertram C. Raynes, Euclid, Ohio, assignor, bymesne assignments, to Horizons Titanium Corporation, Princeton, N. 3., acorporation of New Jersey No Drawing. Application September 30, 1953,Serial No. 383,402

6 Claims. (Cl. 204-39) This invention relates to the cladding of basemetals with corrosion-resistant metals and, more particularly,

to such cladding by electrolysis in a fused salt bath.

The quest for an effective and economical method of producing transitionelements such as titanium and zirconium in pure metallic form has beenspurred by their mechanical properties as well as by theircorrosionresistant properties. Where the advantage in constructing adevice of such a transition element is predicated primarily upon thecorrosion-resistance of the transition element, this objective could beattained by forming the device from a less expensive and more readilyavailable base metal having a surface coating of the corrosion resistanttransition element. A similar coating arrangement would be effectivewhere the strength versus weight characteristics of titanium and thecorrosion-resistance of zirconium are desired. It will be apparent,accordingly, that many of the contemplated applications of thecorrosion-resistant transition elements to structural devices can besubstantially completely satisfied if only the surface of the devicepossesses --the corrosion-resistant characteristic of the transitionelement.

Although a number of metals can be readily electrodeposited on a basemetal in the form of an adherent thin film by aqueous electrolyticmethods, such aqueous electro-dep osition of the transition elementsincluding titanium, zirconium, hafnium, vanaiium, niobium and tantalumhas not been successfully,- chieved. The'classic method ofelectro-depositing these transition elements is through the medium of afused halide salt bath, but it is characteristic of suchelectrodepositionfrom a fused bath that the deposited transition elementis obtained on the cathode in the form of a relatively loosely adheringgranular deposit.

in the copending application of Merle E. Sibert and John T. Burwell,Jr., Serial Number 383,401 filed simul taneously herewith, there isdescribed a method of cladding base metals with the aforementionedtransition elements in the form of a firmly adherent layer of thedeposited metal joined to the base metal by a metal-tometal bond. Theaforesaid method depends upon a combination of a specific range ofrelatively high temperatures for the fused salt bath and a specificrange of cathode current densities in conjunction with the supplying ofthe metal to be deposited by means of a consumable anode comprising asolid metalliferous form of the transition element. Although this methodis effective, it requires relatively high bath temperatures withconsequent complications including volatilization of bath constituents.

l have now found that a base metal may be clad with acorrosion-resistant metal such as the aforementioned transition elementsat considerably lower temperatures by supplying the corrosion-resistantmetal component of the fused salt bath in the form of its alkali metaldouble fluoride and by further incorporating in the bath from about A to10% by weight of water. Thus, my method ited States Patent C ice2,786,809 Patented Mar. 26, 1957 of cladding a base metal with acorrosion-resistant metal by electrolysis in a fused salt bath comprisesestablishing and maintaining a fused salt bath comprising (a) a majoramount of a halide of one or more of the alkali metal and alkaline earthmetal halides, (b) about 15 to 50% by I weight of a double fluoride ofan alkali metal and the corrosion-resistant metal, and (c) about A to10% by weight of water. The bath is maintained at an elevatedtemperature above its melting point, and electrolysis of the molten bathis carried out between a solid anode and a solid cathode composed of abase metal having a melting point substantially above the melting pointof the bath. The base metal cathode is thus clad with thecorrosion-resistant metal by a metal-to-metal bond, and the clad basemetal may be then further fabricated if necessary to shape it for itsintended use.

The base metals which may be clad by the practice of my invention areall metals having a melting point at least as high as 600 C. andinclude, but are not limited to, iron, steel, stainless steels,molybdenum-iron alloys, molybdenum-nickel alloys, nickel-chromiumalloys, nickel-copper alloys, nickel, chromium and copper, and furtherinclude one of the corrosion-resistant metals other than that which isto be deposited, to wit, one of the corrosion-resistant metals titanium,zirconium, hafnium, tantalum, vanadium, niobium, chromium, molybdenumand tungsten. All of the aforementioned base metals have melting pointssubstantially above the melting points of the fused halide salt bathswhich are used in the practice of my invention. The base metal is cladwith the electrodeposited metal by providing the former in the form of acathode having any suitable shape compatible with the geometry of theelectrolytic cell in which it is clad with the corrosion-resistantmetal.

The corrosion-resistant metals which may be deposited on theaforementioned base metals in the form of a firmly adherent layerinclude not only the transition elements titaium, zirconium, hafnium,tantalum, vanadium 'and niobium but also chromium, molybdenum andtungsten. Each of these corrosion-resistant metals may be readilyelectrodeposited from an alkali metal double fluoride of the metal in afused halide salt bath.

The fused salt baths useful in practicing my invention comprise fusedhalide salts composed primarily of one or more of the alkali metalhalides and alkaline earth metal halides, and mixtures thereof. Theuseful halides include the chlorides, bromides, iodides and fluorides,and the choice of specific salt or mixture of salts is wholly dependentupon the desired melting point for the bath. For example, if sodiumchloride is used as the sole constituent of the carrier bath other thanthe alkali metal double fiuodide of the corrosion-resistant metal andthe water, the resulting mixture of the sodium chloride with the doublefluoride will have a melting point Within the range of about 700 C. Onthe other hand, if a eutectic mixture of potassium and sodium chloridesis used, the double fluoride-containing bath will have a melting pointbelow 550 C. Similarly, other bath melting points may be achieved byusing mixtures of the halides of potassium, sodium and lithium and thehalides of calcium, magnesium, strontium and barium. For example,mixtures of calcium and sodium chlorides and mixtures of calcium, sodiumand potassium chlorides have been used satisfactorily as the carriercomponents of the fused salt baths wherein my inventon is practiced. Theaforementioned halide components of the bath comprise a majorconstituent of the bath, the other constituents being generallyrestricted to the double fluoride of the corrosion-resistant metal to beelectrodeposited and a small but significant amount of water.

The alkali metal double fluoride of; the corrosioin resistant metalcomprises the source of the corrosionresistant metal which iselectrodeposited. Although this double fluoride can be produced in situin the bath by electrolysis of a solid metalliferous anode composed ofthe corrosion-resistant metal, I presently prefer to establish thecorrosion-resistant component of the bath by the direct addition theretoof the alkali metal double fluoride. In general, I prefer to use thesodium and potassium double fluorides of the aforementionedcorrosionresistant metals, and i have found that amounts of these doublefluorides ranging from about 15 to 50% by weight of the total bathconstituents are satisfactory in practicing the method of my invention.

The efiective cladding of the corrosion resistant metal on the surfaceof the base metal cathode at fused salt bath temperatures below themelting point of any alloy of the corrosion-resistant metal and the basemetal. of the cathode, pursuant to my invention, is predicated upon thepresence in the bath of a small but significant amount of water. Ingeneral, I have found that from to 10% of water by weight of the totalbath constituents is effective for this purpose, and within this range Ipresently prefer to use from /2.% to 5% of Water by weight of the bath.The Water may be incorporated in the bath either in an indigenous form,such as the water of hydration of one of the bath constituents, or itmay he added from an extraneous source.

The effectiveness of the water component of the bath in promotingcladding pursuant to my invention appears to be a resultof its presenceas water and not by virtue of any oxide or oxides which may be formed inthe bath as a result of the: presence of the water inthe hot halides.The presence of suchoxides, in the absence of a significant amount ofWater pursuant to my present invention, appears to be wholly ineffectivein. promoting the formation of a metal-to-metal bond between theelectrodeposited corrosion-resistant metal and the base metal cathode.However, the presence of such an extraneous oxide in a bath containinga. significant amount of water in accordance with the invention does notadversely affect thequality of. the cladding bond nor does it appear tohave any pronounced advantageous effect upon the cladding.

The cladding operation is performed simply by electrolyzingtheaforementioned bath composition between a solid anode and a solidcathode composed of the base metal to be clad; This electrolysis takesplace readily at cell voltages of at least 4.5 volts and preferably ateelii voltages within the range of about'S to 8 volts. The choice ofcell voltage with-in this range is based upon the cell geometry and uponthe shape and size of the cathode, the voltage being such as toestablish and maintain a cathode current density Within the range ofabout 100 to 500 amp'eres per square decimeter. The cell atmosphereabove the bath should be maintained free of oxygen, nitrogen and carbon,and for this purpose I have found it suitable to maintain either vacuumconditions above the molten bath or tos-Weep the cell with an inert gassuch as argon. In general, the cell operation may be continued withoutinterruption for a period sufiicient to effect the desired cladding ofthe base metal cathode with an imperviouslayerof corrosion-resistantmetal, but when theamount of water initially present in the bath mixtureis low it has been found to be advantageous in some instances toreplenish the water component of the bath either continuously orintermittently during the electrolysis. The electrolytic cell design andstructure are not critical, although due care should be taken to usecell components which are not attacked by the fused salt bath duringelectrolysis. Graphite is wholly satisfactory as a cell-wall material,and, the graphite cell either may be used as the anode or it may remainelectrically neutral whereupon a graphite rod or plate (or other solidmaterial not attacked by chl'orine' at the prevailing cell temperature)is used asthe anode;

After cladding of the cathode base metal has been effected, the cathodeis withdrawn from the bath and into a cooling chamber associated withthe cell where it is permitted to cool to room temperature inan inertatmosphere without being exposed during the interim to the ambientatmosphere. The cooled cathode is then removed from the cooling chamberand the salt cake which has frozen around the metal is leached byconventional means so as to expose the clad layer.

The following specific examples are illustrative of the practice of myinvention:

Example 1 Six hundred parts by weight of potassium fiuozirconate and 12parts by weight of water were mixed with 1800 parts by weight of sodiumchloride. The resulting mixture was then placed in a graphite crucibleand was heated and maintained at at temperature of 750 to 850 C. in anatmosphere of argon. After fusion of the salt mixture occured an ironbase metal shape was completely immersed in the fused bath. A voltage ofabout 6 volts was applied between the praphite crucible as the anode andthe base metal shape as the cathode in order to maintain a cathodecurrent density of about 420 amperes per square decimeter. Theelectrolysis was continued for 20 minutes,- at the end of which period adeposit of metallic zirconium $5 inchthick was obtained on the basemetal shape. Upon completion of the electrolysis, the cathode shapewas-withdrawn from the bath and cooled in an argon atmosphere to roomtemperature. The clad piece was removed from thecooling chamber and wasleached free of residual salts. Metallographie examination of theresulting product showed that the zirconium-base metal bond Wascontinuous and not intermittent, the base metal adjacent the bondexhibiting an intermediate region approximately equal in thickness tothat of the zirconium layer.

Example H The fused salt bath of sodium chloride and potassiumfluozirconate described in Example I but with no addition of waterfailed to produce a clad layer of zirconium at bath temperatures of 770800 C. when attempts were made to deposit the zirconium on cathodes ofsteel, ingot iron, molybdenum, nickel and copper in the same cell andunder the same electrolysis conditions prevailing in Example I.

Example III Example IV A fused bath consisting of 1800 parts by weightof sodium chloride and 400- parts by weight of potassium fiuotitanate,the'latter containing'20% Water of crystallization, was electrolyzed at77 0 C. under the cell conditions set forth in Example I and resulted ina clad layer of titanium adhering'to'an ingot iron cathode by ametalto-me'tal bond.

It will be seen, accordingly, that the method of my inventionefiectivelyclads a wide variety of base metals with a firmly adherentlayer of a corrosion-resistant metal. Inasmuch as the resulting productmay frequently be used :without further deformation during subsequentfabrication, the ductility of the clad layer is not important. As amatterrof fact, the tendency of the electrodeposited metal to exhibit ahardness greater than that compatible with cold rollability, as isgenerally exhibited by these metals in their electrodeposited form,becomes a virtue inasmuch as it enhances the durability of thecorrosionresistant surface on the basic metal object. Thus, the purityof the clad layer is relatively unimportant except for any influencewhich such impurities may have on the corrosion-resistance of thecladding metal. Nevertheless, clad layers produced in the manner of myinvention have exhibited fabrication properties which permit the cladspecimens to be deformed readily in subsequent fabrication.

I claim:

1. The method of cladding a base metal with a corrosion-resistant metalof the group consisting of titanium, zirconium, hafnium, vanadium,tantalum, niobium, chromium, molybdenum and tungsten by electrolysis ina fused salt bath which comprises establishing a fused salt bathconsisting essentially of (a) a major amount of a halide of the groupconsisting of alkali metal and alkaline earth metal halides, (b) about15 to 50% by weight of a double fluoride of an alkali metal and thecorrosion-resistant metal and (c) about A to 10% by weight of water,maintaining said bath at an elevate-d temperature above its meltingpoint, and then electrolyzing the bath at said elevated temperaturebetween a solid anode and a solid cathode composed of a base metalhaving a melting point substantially above the melting point of thebath, and recovering the resulting base metal cathode clad by ametalto-metal bond with the corrosion-resistant metal.

2. The method of cladding a base metal with a corrosion-resistant metalof the group consisting of titanium, zirconium, hafnium, vanadium,tantalum, niobium, chromium, molybdenum and tungsten by electrolysis ina fused salt bath which comprises establishing a fused salt bathconsisting essentially of (a) a major amount of a halide of the groupconsisting of alkali metal and alkaline earth metal halides, (b) about15 to 50% by weigh of a double fluoride of an alkali metal and thec-orrosiomresistant metal and about /1 to 5% by Weight of water,maintaining said bath at an elevated temperature above its meltingpoint, and then electrolyzing the bath at said elevated temperaturebetween a solid anode and a solid cathode composed of a base metalhaving a melting point substantially above the melting point of thebath, and recovering the resulting base metal cathode clad by ametal-to-metal bond with the corrosion-resistant metal.

3. The method of cladding a base metal with a corrosion-resistant metalof the group consisting of titanium, zirconium, hafnium, vanadium,tantalum, niobium, chromium, molybdenum and tungsten by electrolysis ina fused salt bath which comprises establishing a fused salt bathconsisting essentially of (a) a major amount of a halide of the groupconsisting of alkali metal and alkaline earth metal halides, (b) about15 to 50% by weight of a double fluoride of an alkali metal and thecorrosion-resistant metal and (0) about M; to by weight of water,maintaining said bath at an elevated temperature above its melting pointbut below the melting point of any alloy of the corrosion-resistantmetal with the base metal, and then electrolyzing the bath at saidelevated temperature between a solid anode and a solid cathode composedof a base metal having a melting point substantially above the meltingpoint of the bath, and recovering the resulting base metal cathode cladby a metabto-metal bond with the corrosionqresistant metal.

4. The method of cladding a base metal with a C01 rosion-resistant metalof the group consisting of titanium, zirconium, hafnium, vanadium,tantalum, niobium, chromium, molybdenum and tungsten by electrolysis ina fused salt bath which comprises establishing a fused salt bathconsisting essentially of (a) a major amount of a halide of the groupconsisting of alkali metal and alkaline earth metal halides, (b) about15 to 50% by weight of a double fluoride of an alkali metal and thecorrosionresistant metal and (0) about Mr to 10% by weight of Water,maintaining said bath at an elevated temperature above its meltingpoint, and then electrolyzing the bath at said elevated temperaturebetween a solid anode and a solid cathode composed of a base metalhaving a melting point substantially above the melting point of the bathand with a cathode current density within the range of about to 500amperes per square decimeter, and recovering the resulting base metalcathode clad by a metalto-metal bond with the corrosion-resistant metal.

5. The method of cladding a base metal of the group consisting oftitanium, zirconium, hafnium, vanadium, tantalum, niobium, chromium,molybdenum and tungsten with another metal of said group by electrolysisin a fused salt bath which comprises establishing a fused salt bathconsisting essentially of (a) a major amount of a halide of the groupconsisting of alkali metal and alkaline earth metal halides, (b) about15 to 50% by weight of a double fluoride of an alkali metal and of themetal with which the base metal is to be clad, and (c) about A to 10% byWeight of water, maintaining said bath at an elevated temperature aboveits melting point, and then electrolyzing the bath at said elevatedtemperature between a solid anode and a solid cathode composed of a basemetal having a melting point substantially above the melting point ofthe bath, and recovering the resulting base metal cathode clad by ametal-to-metal bond with the corrosion-resistant metal.

6. The method of cladding a base metal with a corrosion-resistant metalof the group consisting of titanium, zirconium, hafnium, vanadium,tantalum, niobium, chromium, molybdenum and tungsten by electrolysis ina fused salt bath which comprises establishing a fused salt bathconsisting essentially of (a) a major amount of a halide of the groupconsisting of alkali metal and alkaline earth metal halides, (b) about15 to 50% by weight of a double fluoride of an alkali metal and thecorrosionresistant metal and (0) about A to 10% by weight of water,maintaining said bath at an elevated temperature above its melting pointunder a cell atmosphere substantially free of oxygen-, nitrogenandcarbon-containing gases, electrolyzing the bath at said elevatedtemperature between a solid anode and a solid cathode composed of a basemetal having a melting point substantially above the melting point ofthe bath, and recovering the resulting base metal cathode clad by ametal-to-metal bond with the corrosion-resistant metal.

References Cited in the file of this patent UNITED STATES PATENTS1,535,339 Peacock Apr. 28, 1925 1,845,978 Hosenfeld Feb. 16, 19321,933,319 Driggs et a1. Oct. 31, 1933 2,715,093 Senderofi et a1. Aug. 9,1955

1. THE METHOD OF CLADDING A BASE METAL WITH A CORROSION-RESISTANT METALOF THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, HAFNIUM, VANADIUM,TANTALUM, NIOBIUM, CHROMIUM, MOLYBDENUM AND TUNGSTEN BY ELECTROLYSIS INA FUSED SALT BATH WHICH COMPRISES ESTABLISHING A FUSED SALT BATHCONSISTING ESSENTIALLY OF (A) A MAJOR AMOUNT OF A HALIDE OF THE GROUPCONSISTING OF ALKALI METAL AND ALKALINE EARTH METAL HALIDES, (B) ABOUT15 TO 50% BY WEIGHT OF A DOUBLE FLOURIDE OF AN ALKALI METAL AND THECORROSION-RESISTANT METAL AND (C) ABOUT 1/4 TO 10% BY WEIGHT OF WATER,MAINTAINING SAID BATH AT AN ELEVATED TEMPERATURE ABOVE ITS MELTINGPOINT, AND THEN ELECTROLYZING THE BATH AT SAID ELEVATED TEMPERATUREBETWEEN A SOLID ANODE AND A SOLID CATHODE COMPOSED OF A BASE METALHAVING A MELTING POINT SUBSTANTIALLY ABOVE THE MELTING POINT OF THEBATH, AND RECOVERING THE RESULTING BASE METAL CATHODE CLAD BY AMETALTO-METAL BOND WITH THE CORROSION-RESISTANT METAL.